During the course of assembly of relative parts it is often necessary to ensure that the interface loading is at a specific stress level. One of the current methods to achieve this is to use bolts or studs with nuts and washers, torqued to a predetermined level in order to impose the necessary loading. This method assumes that the torque imparted via calibrated torque wrenches or similar devices will impart loading to the respective interface to achieve the calculated stress levels. This method is quick and universally accepted, but the resultant loading is very dependent upon the thread profile and the coefficient of friction between the shank and the nut, as well as the interface coefficient of friction between the nut and the part being loaded.
A variety of apparatus exists to impart loading into a nut and bolt or stud and nut connector system, but all of them require torque to be imparted to the nut. With the slight variations of profile and surface finish the actual loading that is imparted axially to the bolt or stud varies from bolt to bolt even though a calibrated torque wrench or strain gauge loading device is used to impart the torque. This is recognized throughout the industry, and a number of techniques have been developed to measure the elongation of the bolt or stud. Such techniques use direct axial change in length measurement criteria to determine the strain in the bolt or stud. Such measurement apparatus ranges from the use of dial indicator gauges, ultra sonic measurement, angular measurement, and interferometer techniques. All such methods require the attachment of external devices or pre/post stress measuring techniques, which take time and are in themselves subject to error.